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Although the relative importance of β-amyloid plaques and tau protein tangles in the progression of Alzheimer’s disease has been the subject of much debate, early emphasis was placed on the development of drugs to block production of β-amyloid. Although such compounds were shown to improve cognition in transgenic mice, unfortunately results from clinical trials have been more equivocal. Focus is now shifting to therapies that target tau pathology and, in a recent study, researchers from the University of Pennsylvania have identified a compound that reduced cognitive deficits in mutant human tau transgenic mice.

In healthy nerve cells, tau proteins interact with tubulin to stabilize axonal microtubules and promote tubulin assembly into microtubules. In Alzheimer’s disease and other ‘tauopathies’, hyperphosphorylated and misfolded tau proteins form insoluble neurofibrillary tangles that deplete levels of soluble tau and lead to destabilization of the microtubules and neuronal dysfunction. The team had previously proposed using microtubule-stabilising anti-cancer taxanes such as paclitaxel to treat tauopathies, but these do not penetrate the blood-brain barrier sufficiently well. The Penn team has now shown that once weekly treatment of tau transgenic mice with the brain-penetrant microtubule-stabilising agent, epothilone D, for three months significantly improved microtubule density and axonal integrity and also reduced cognitive deficits without notable side-effects.

The study, which is published in the Journal of Neuroscience, suggests that brain-penetrant microtubule-stabilising drugs could provide a new strategy for treating Alzheimer’s disease.

Epidemiological studies have suggested that either rheumatoid arthritis itself – or the anti-inflammatory drugs used to control it – are associated with a reduced risk of developing Alzheimer’s disease. Recent clinical trials with non-steroidal anti-inflammatory drugs (NSAIDs) have failed to show a benefit in patients with Alzheimer’s disease and researchers at the University of South Florida have now shown that it may be the disease itself that affords protection.

In a mouse model of Alzheimer’s disease, one of the cytokines that is elevated in patients with rheumatoid arthritis, granulocyte-macrophage colony-stimulating factor (GM-CSF), was shown to improve working memory and learning and to lead to an apparent increase in neural cell connections in the animals’ brains. GM-CSF also led to an accumulation of microglia in the brains of treated animals which was associated with a greater than 50% reduction in β-amyloid peptides. Recombinant human GM-CSF is currently approved to stimulate the production of white blood cells in some cancer patients and the new study, which is published in the Journal of Alzheimer’s Disease, suggests that GM-CSF could also be of benefit to Alzheimer’s patients.

The USF Health Byrd Alzheimer’s Institute plans to begin a pilot clinical trial later this year to investigate recombinant human GM-CSF in patients with mild or moderate Alzheimer’s disease.

Following from positive phase II results, the announcement earlier this year that Dimebon (latrepirdine) failed to show a significant effect in a phase III clinical trial in Alzheimer’s patients was a major blow to patients, families and doctors. A study by researchers at UT Southwestern Medical Center has now shown that Dimebon can increase neurogenesis in adult rodent brains and have identified other, more potent compounds.

An in vivo screen of 1000 small molecules in adult mice identified eight compounds that were able to enhance neuron formation in the subgranular zone of the hippocampal dentate gyrus. One of the compounds, P7C3, was selected for further study on the basis of favourable ADME predictions. Daily administration of P7C3 to aged rats for 7 days was shown to enhance hippocampal neurogenesis relative to control animals and, after 2 months, treated rats performed significantly better in the Morris water maze test which provides a measure of learning and memory.
P7C3 exerts its proneurogenic effects by protecting newborn neurons from apoptosis and the team next compared the activity of P7C3 with that of Dimebon, which is also believed to have anti-apoptotic activity. Dimebon was found to be proneurogenic in vivo, albeit at levels 10-30 times higher than P7C3, raising the possibility that the two compounds may share a common mechanistic pathway. Although this idea can only be rigorously tested after identification of the molecular target(s), the study raises the hope that more potent analogues of Dimebon with improved clinical efficacy could be identified and also provides appropriate assays.

The cognitive improvement in Alzheimer’s disease patients brought about by treatment with acetyl cholinesterase inhibitors has been largely attributed to enhanced M1-muscarinic receptor signalling. Recently, however, studies with M1-receptor knockout mice and with more selective M1-receptor modulators have suggested that this receptor may not directly mediate learning and memory.

A team led by researchers at the University of Leicester has now suggested an alternative mechanism involving the M3-muscarinic receptor which is widely expressed in many brain regions, including the hippocampus. M3-receptor knockout mice were found to show a deficit in fear conditioning learning and memory. A knock-in mouse strain expressing a phosphorylation-deficient receptor also showed a deficit in fear conditioning learning, indicating that the learning process involves receptor phosphorylation. Agonist treatment and fear conditioning training led to phosphorylation of the M3-receptor in the hippocampus, confirming the importance of receptor phoshorylation on learning and memory. The phosphorylation-deficient receptor was expressed normally at the cell surface and was able to signal via the Gq/11 calcium pathway, but was uncoupled from phosphorylation-dependent processes such as receptor internalization and arrestin recruitment. The study, which is published in PNAS, suggests that an M3-receptor modulator that enhances phosphorylation/arrestin-dependent (non-G protein) signalling may be beneficial in treating cognitive disorders. ‘Biased’ ligands – those able to direct signalling of GPCRs selectively through the phosphorylation/arrestin-dependent pathway – have recently been described for a number of other GPCRs.

ApoE is a lipid transport protein with roles in transport of dietary lipids, regulation of plasma cholesterol, and protection from atherosclerosis. In humans, there are three variants of ApoE (ApoE2, ApoE3 and ApoE4) and one of these, ApoE4, has been linked to earlier onset of Alzheimer’s disease. The mechanisms underlying the increased risk of Alzheimer’s disease remain unclear but researchers at UT Southwestern Medical Center have now shown that the ApoE4 variant reduces surface expression of receptors involved in synaptic plasticity by sequestering the receptors inside the cell.

ApoE interacts with members of the LDL receptor family and one of the receptors for ApoE, Apoer2, also acts as a signalling receptor for reelin, a protein that is important in the developing brain but also enhances NMDA receptor activity and increases long-term potentiation (LTP) in the adult brain. ApoE4 was found to reduce surface expression of NMDA and AMPA receptors as well as Apoer2 receptors, thereby impairing glutamatergic neurotransmission. β-Amyloid peptide, a hallmark of Alzheimer’s disease, suppresses LTP and the ability of reelin to counter the effects of β-amyloid peptide was almost completely abolished in mice expressing human ApoE4. The team are now trying to understand whether it is possible to build on their findings to develop new treatments for Alzheimer’s disease.

Rapamycin was first isolated from soil samples taken from Rapa Nui (Easter Island)

Image: Flickr - A.www.viajar24h.com

Two recent reports from the University of Texas Health Science Center suggest that rapamycin may find a new use in treating Alzheimer’s disease (AD). The first study, published in the Journal of Biological Chemistry, showed that treatment with rapamycin reduced AD-like pathology in 3xTg-AD mice and the more recent study, published in PLoS ONE, showed that rapamycin also improved spatial learning and memory in PDAPP transgenic mice, another murine model of AD. Earlier studies had shown that rapamycin, which blocks the mTOR pathway and is used as an immunosuppressant to prevent rejection following organ transplants, can extend the lifespan of mice, even if treatment is started relatively late in life.

In both 3xTg-AD and PDAPP mice, treatment with rapamycin was found to lower levels of amyloid peptide, Aβ42, a hallmark feature of AD which is believed to be a major cause of neurotoxicity. The reduction in Aβ42 when mTOR activity is reduced may be a consequence of induction of autophagy by high levels of Aβ42. The improved performance of PDAPP mice treated with rapamycin appeared to result entirely from enhanced cognitive performance with no effect on emotions such as anxiety.

The authors suggest that, if the results with mice are borne out by studies in people, rapamycin could be used to prevent or treat AD but the potential benefits will need to be carefully weighed against increased susceptibility to infection and the possible development of some types of malignancy which could result from prolonged immunosuppression.

Conventional wisdom, supported by in vitro experiments, has previously suggested that phosphoinositide 3-kinase (PI3K) plays a protective role in Alzheimer’s disease. However, a team led by researchers at Cold Spring Harbor has now implicated PI3K in the pathogenesis of the disorder.

The team used fruit-flies (Drosophila) that were engineered to produce human β-amyloid in their brains – a model that mimics many of the features of Alzheimer’s, including age-dependent memory loss, neurodegeneration, β-amyloid deposits and plaque formation. In the model, the presence of β-amyloid enhances long term depression (LTD), a process in which nerve signal transmission at particular synapses is depressed for an extended period. The research demonstrated that the enhanced LTD was a consequence of increased PI3K activity and could be abrogated by genetic silencing or pharmacological inhibition of PI3K.

The researchers note that the up-regulation of PI3K may also explain the insulin-resistance observed in the brains of Alzheimer’s victims. Insulin is one of the molecules that normally induce PI3K activity, which in turn mediates the cell’s response to insulin. Since PI3K is already hyperactivated in response to β-amyloid, it may no longer be able to respond to insulin.

Roles have been suggested for brain-derived neurotrophic factor (BDNF) – which helps to support neurons and also stimulates and controls neurogenesis – in preventing or treating degenerative diseases such amyotrophic lateral sclerosis, Parkinson’s disease, and Alzheimer’s disease. The use of BDNF itself in therapy is limited by a poor pharmokinetic profile including rapid metabolism and poor CNS penetration. BDNF elicits at least some of its effects through binding to the high affinity tyrosine kinase receptor B, TrkB, and investigators at Emory University School of Medicine have now identified a small, high-affinity molecule that can also activate signalling through TrkB.

7,8-dihydroxyflavone

7,8-Dihydroxyflavone was shown to protect wild-type, but not TrkB-deficient, neurons from apoptosis. Following intraperitoneal administration, the compound was also found to activate TrkB in the brain and to be protective in animal models of seizure, stroke and Parkinson’s disease. The compound was also found to have low toxicity on chronic dosing. Although favonoids such as 7,8-dihydroxyflavone occur in a wide range of foodstuffs, levels obtained from a normal diet are believed to be insufficient for a sustained effect.

Neurofibrillary tangles (NFT) are a hallmark of Alzheimer’s disease (AD) and correlate strongly with synaptic loss and severity of dementia. NFT appear to be attributable, at least in part, to hyperphosphorylation of the microtubule-stabilising protein, tau. Numerous phosphorylation sites have been associated with tau dysfunction and neurodegeneration: phosphorylation on Ser262 has been shown to occur early in disease progression and to significantly reduce the affinity of tau for microtubules. Although hyperphosporylation at other sites is likely necessary for neurodegeneration, increased phosphorylation at Ser262 is an important early step and firm identification of the kinase(s) responsible for phosphorylation at this position could provide new targets for disease-modifying treatments. Numerous kinases have been reported to phosphorylate tau at Ser262in vitro, but the role of these kinases on neurofibrillary tangle formation in vivo remains unclear.

Using a loss of function high throughput RNAi approach to screen the entire human kinome, a team led by scientists at the Translational Genomics Research Institute (TGen) have now identified three new kinases, dual specificity tyrosine phosphorylation-regulated kinase 1A (DYRK1A), A-kinase anchor protein 13 (AKAP13), and eukaryotic translation initiation factor 2-alpha kinase 2 (EIF2AK2) that contribute to hyperphosphorylation of tau. Whereas DYRK1A and AKAP13 appear to be specifically involved in tau phosphorylation pathways, the effects of EIF2AK2 may result from alterations in tau protein expression.

If further studies in neuronal cell lines and in vivo models of AD and tauopathies confirm the importance of these kinases in disease pathology, they would represent novel targets for disease-modifying treatments for AD.

Prion diseases comprise the transmissible spongiform encephalopathies, including scrapie in sheep, bovine spongiform encephalopathy (BSE, “Mad Cow” disease) in cattle and Creutzfeldt-Jakob disease in humans. Central to these diseases is the conversion of normal cellular prion protein (PrPc) into the abnormally folded, pathogenic species (PrPSc) in the brain. The misfolding results in prion protein with distinct biochemical properties compared to the normal protein, such as reduced solubility and decreased susceptibility to proteases. Aggregates of PrPSc accumulate in association with neurons in affected brain areas, which is thought to lead to the synapse degeneration and neuronal death observed in infected hosts.

Glimepiride

Researchers in the UK and Italy have now shown that glimepiride, a sulfonyl urea approved for the treatment of non insulin dependent diabetes mellitus (NIDDM), is able to reduce PrPSc formation in cell culture. The rationale for the study was based on the knowledge that generation of PrPSc is dependent on the presence of PrPc and that this appeared to require PrPc expressed at the cell surface. PrPc is linked to the membrane by a glycosylphosphatidylinositol (GPI) anchor and can be released from the surface of cells by treatment with phosphatidylinositol-phospholipase C (PI-PLC). Consistent with the hypothesis that cell-surface PrPc is required, treatment of prion-infected neuronal cells with PI-PLC reduced PrPSc formation.

Since glimepiride has been shown to stimulate the release of some GPI-anchored proteins in adipocytes (via stimulation of an endogenous GPI-PLC), the team explored the effects of the drug on PrPc/PrPSc in neuronal cell culture. Similarly to PI-PLC, glimepiride reduced the amount of cell-surface PrPc in primary cortical neurons and neuronal cell lines. In addition, glimepiride reduced formation of PrPSc in three prion-infected neuronal cell lines.

The study, published in PLoSone, also demonstrated that glimepiride treated neurons were resistant to the toxicity of a PrP-derived peptide, PrP82-146.

The team note that modulation of cell-surface PrPc may also have application in Alzheimer’s disease since it is a receptor for β-amyloid oligomers. Whether glimepiride is sufficiently CNS-penetrant to be effective remains to be seen.